Article
A leaking flange is rarely caused by the flange alone. Most leaks come from a combination of small problems in the gasket, the facing, the bolts or the way the joint was assembled. Here are the causes seen most often in plants, and how to avoid them.
1. Wrong gasket type or size
A gasket rated for Class 150 used on a Class 300 flange, or a non-metallic gasket at high temperature, will fail. Match gasket type, material, size and class to the flange and the service.
2. Damaged or wrong surface finish
Radial scratches across a raised face create direct leak paths. Spiral wound gaskets generally need a serrated finish around 125–250 µin Ra; very smooth faces can let them slip. Inspect and protect faces before installation.
3. Misalignment
If the pipes do not line up, the flanges are forced together with the bolts and the gasket is loaded unevenly. Correct the piping or supports instead of pulling the joint together with bolt force.
4. Uneven or insufficient bolt load
Tightening bolts in a circle, in one pass, or with no lubrication gives uneven gasket stress. Use the star pattern, staged passes and calibrated tools.
5. Wrong or mixed bolting
Bolts that are too weak for the class, or a mix of grades, cannot hold the required load. Use the specified grade, such as ASTM A193 B7 with A194 2H nuts, or B8/B8M for stainless.
6. Mismatched flanges
Class 150 and Class 300 flanges, or RF and FF flanges, do not match. Mismatches in bore, facing or drilling must be caught at receipt inspection.
7. Thermal cycling and relaxation
Temperature swings relax gaskets and bolts. In hot service, plan a hot torque check and choose gaskets suited to cycling.
8. Corrosion and erosion
Crevice corrosion under the gasket or erosion at the bore slowly destroys the sealing surface. Choose flange and gasket materials suited to the fluid.
Quick prevention checklist
| Check | Before assembly |
|---|---|
| Flange faces | Clean, no radial scratches, correct finish |
| Gasket | New, correct type, size and class |
| Bolts | Correct grade and length, lubricated |
| Alignment | Faces parallel, bolt holes aligned freely |
| Bolting | Star pattern, staged passes, final check |
Frequently asked questions
What is the most common cause of flange leaks?
Uneven or insufficient gasket compression, usually from poor bolting practice or misalignment.
Can a scratched flange face be used?
Small circumferential marks may be acceptable, but radial scratches across the sealing area should be repaired by machining.
Should gaskets be reused?
No. Fit a new gasket every time a joint is opened.
Key takeaways
- Most leaks come from gasket, facing, alignment or bolting problems.
- Inspect faces and use a new, correct gasket every time.
- Follow a star pattern with staged torque.
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Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.